A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) of a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus may be of the transmissive type, where radiation is passed through a mask to generate the pattern, or of the reflective type, where radiation is reflected from the mask to generate the pattern. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the scanning-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In general, there is non-uniformity in the intensity of radiation which is imaged onto the substrate in such apparatus. This is typically caused by, for example, the mirrors or lenses of the illumination and projection system having differing reflectivity or transmission over their surfaces. In the case of conventional lithography, so-called deep-UV (DUV) used with wavelengths of 248 nm or less, a transmissive filter is included which corrects for this non-uniformity. In the past the properties of the filter were fixed and could not be changed over time. In newer systems the filter is adjustable, and can be adjusted to take account of slow variations in beam uniformity, for example caused by gradual degradation of lens surfaces.
A known adjustable uniformity correction unit for DUV comprises two transmissive plates that are considerably bigger than the projection beam. Different transmission profiles are provided on the plates, so that, when the transmission of the plate is to be adjusted, the point at which the projection beam intercepts the plate is changed by moving the plate.
Furthermore it is known that the intensity of the radiation reaching the substrate increases over time as the apparatus warms up in use. This is because the transmission of the optical elements increases as the optical elements get warmer. U.S. Pat. No. 6,455,862 discloses a software model that compensates for this increase on the basis of measurements made with energy sensors at wafer level before the production run, the results of such measurements being inputted to the model which is then used during the production run to adjust the intensity of radiation incident on the substrate so that the intensity of radiation received by the substrate remains substantially constant over time.
In extreme ultraviolet (EUV) lithography, there are no materials available which can be used in a transmissive way. Accordingly an arrangement is disclosed in U.S. Pat. No. 6,741,329 in which non-transmissive blades, commonly called luxaflex blades, are used to adjust the beam to correct for non-uniformity in the intensity of radiation imaged onto the substrate. The blades are in the form of a series of parallelograms that are rotatably mounted and are spread across the projection beam. In order to reduce the beam intensity in a given location, the blade at that location is rotated so that it partially blocks the beam. The blades are typically located 90 mm below the reticle. If the blades were to be located further away from the reticle, then sharp images of the blade edges would appear on the substrate. Conversely, if the blades were to be moved nearer to the reticle, then the spatial frequency of the intensity correction provided by the blades would be reduced.
Because the illumination slit that is used to expose the substrate during scanning is usually curved, the orientation of the blades with respect to the slit is not constant. For example, at the left hand end of the slit the blades may be mounted at 30° relative to the scanning direction, whereas at the right hand side of the slit the blades may be mounted at 45° relative to the scanning direction.
In addition to the time-varying average transmission of the optical elements referred to above, the optical elements also cause a shaped intensity profile at the illumination slit dependent on the particular apparatus used. This can be compensated for by providing a fixed transmission compensation optical element. However it has been observed by the applicant that the shape of the intensity profile at the illumination slit also changes with time, as a result of heating of the optical elements. There is therefore a need to be able to compensate for variation of the shape of the intensity profile with time.